Establishing literacy in science is often linked to building knowledge about the Nature of Science (NOS). This paper describes and evaluates an inservice program designed to build elementary teachers' understanding of NOS and an awareness of how NOS impacts science classroom instruction. Data sources consisted of surveys, action research plan documentation and classroom observations. Program participants tended to demonstrate some gains in understanding more about NOS and they linked positive experiences in the program to the explicit and activity-based NOS instruction provided. Yet, participation in the professional development project might not have been equally beneficial for all teachers. The understanding of NOS may have been restricted to certain NOS aspects, and the demonstration of the participants' understanding of NOS may have been short-lived with a somewhat limited impact on sustainable, long-term NOS-based classroom instruction. Implications for designing NOS related professional development programs and suggestions for improvements to further develop teacher understanding of NOS are discussed.